1. Field of the Invention
The present invention relates to an image recording apparatus having a detection mechanism which detects an ejection state of a nozzle.
2. Description of the Related Art
An ink jet type image recording apparatus records an image by ejecting an ink to a recording medium. The image recording apparatus has a recording head which ejects an ink to the recording medium, a carriage which holds the recording head, transferring means for transferring the recording medium, and carriage driving means for moving the carriage in a direction (main scan direction) orthogonal to a transferring direction (sub scan direction) of the recording medium by the transferring means. Further, the recording head has a plurality of nozzles which are ejection openings for the ink.
The image recording apparatus drives the carriage along the main scan direction. The recording head is moved along the main scan direction by drive of the carriage. During this movement, the recording head injects ink droplets to the recording medium. Specifically, the image recording apparatus sequentially ejects the ink from each of a plurality of the nozzles during movement of the carriage. By doing so, the image recording apparatus sequentially records a plurality of ink dots on the recording medium. The image recording apparatus forms a desired image with these ink dots.
Usually, the image recording apparatus records the ink dots D with the even arrangement as a whole as shown in FIG. 12. It is to be noted that reference character MD denotes the main scan direction and reference character SD designates the sub scan direction in FIG. 12.
As shown in FIG. 12, the respective ink dots D are recorded at intervals dl along the sub scan direction SD. Furthermore, the respective ink dots D are recorded at intervals dw along the main scan direction MD. Moreover, the interval dl and the interval dw are substantially equal to each other. Therefore, the formed image has the uniform density distribution as a whole. It is to be noted that the interval dl is determined based on a gap between the respective nozzles of the recording head. The interval dw is determined based on a moving velocity of the recording head in the main scan direction and an ejection timing of the ink. Actually, however, since an ejection cycle of the ink is determined based on the essential capability of the recording head, it is difficult to reduce the interval beyond the capability. Therefore, the interval dw is usually determined based on the moving speed of the recording head. Thus, in the regular image recording apparatus, a fixed moving speed is set in order to evenly arrange the ink dots as mentioned above. Concretely, in the image recording apparatus, the moving speed of the recording head in the usual image recording mode is set to a speed that the recording head can move for a distance corresponding to the interval dl of the nozzles in one cycle of the ejection cycle of the ink.
The quality of an image obtained by the ink dots is deteriorated when an ejection defect is generated due to clogging of the nozzles or the like.
Therefore, an image recording apparatus having a detection mechanism which detects the ejection defect is proposed. There are mainly two modes of the detection mechanism. The detection mechanism of the first mode performs test printing on the recording medium, and detects an ejection defect by reading a test-printed image by using a scanner.
The detection mechanism of the second mode has a light source and a photo detecting element which receives a beam from the light source. The light source is arranged in such a manner that the ink droplet ejected from the recording head can be transmitted through the beam. The detection mechanism of the second mode detects an ejection defect by detecting a change in quantity of received light in the photo detecting element when the ink droplet has been transmitted through the beam.
Since the detection mechanism of the second mode does not require a scanner moving time and an image reading time which are necessary in the detection mechanism of the first mode, it can detect the ejection defect at a higher speed. The conventional image recording apparatus of the second mode is constituted as shown in FIG. 13A, for example.
In FIG. 13, the image recording apparatus 110 has a recording head 120, a carriage 130 which supports the recording head 120, transferring means 140 for transferring a recording medium P in the sub scan direction, and driving means 150 for driving the carriage 130 in the main scan direction. In addition, the image recording apparatus 110 also has the above-described detection mechanism 160. The recording head 120 has a plurality of nozzles 121 which are arranged so as to face the recording medium P during image recording. The nozzles 121 are ink ejection openings.
The detection mechanism 160 is arranged outside of an image recording area in which an image is recorded in the main scan direction. In other words, the detection mechanism 160 is arranged in an inspection area which is an area other than the image recording area. The inspection area is an area used for detecting an ejection defect of the recording head. The detection mechanism 160 has an ink reservoir 161, a light source 162 and a photo detector 163. The ink reservoir 161 receives the ink ejected in the inspection area. Therefore, the ink reservoir 161 prevents the inside of the apparatus from being stained by the ejected ink when detecting an ejection defect.
The light source 162 is arranged along the arrangement direction of the nozzles 121 of the recording head 120 which has moved in the inspection area so as to be capable of emitting a beam. In other words, the detection mechanism 160 has an optical axis along the arrangement direction of the nozzles. The beam is schematically pointed by reference character B in FIG. 13B.
The photo detector 163 has a photo detecting element and is arranged so as to be capable of receiving the beam B from the light source 162.
The image recording apparatus 110 having the detection mechanism 160 detects an ejection defect as follows. The recording head 120 is first moved into the inspection area upon movement of the carriage 130 due to drive by the driving means 150. It is to be noted that the recording head 120 is moved in such a manner that the nozzles 121 are arranged on the optical axis of the beam B in the main scan direction as shown in FIG. 13C. That is, each nozzle 121 is arranged at a position intersecting the optical axis.
The recording head 120 causes the respective nozzles 121 from the nozzle 121 on one end side of the recording head to the nozzle 121 on the other end side of the same to sequentially eject the ink in the inspection area. At this moment, the ejected ink droplets are sequentially transmitted through the beam B and spotted in the ink reservoir 161. Since a quantity of received light varies when the ink droplet passes through the beam B, the photo detector 163 can detect passage of the ink droplet.
However, the detection mechanism 160 must match the optical axis of the beam B with the arrangement direction of the nozzles 121 in order to detect an ejection defect. Therefore, movement of the recording head 120 must be highly accurately controlled. Therefore, the detection mechanism 160 and the driving means 150 disadvantageously become complicated mechanisms. In addition, the image recording apparatus 110 performs inspection while carriage of the recording head 120 is stopped. Therefore, the entire image recording time including the inspection time of the image recording apparatus 110 is increased. That is, a recording speed of an image including the inspection time in the image recording apparatus 110 is lowered.
In the image recording apparatus including the detection mechanism, various proposals are provided in order to overcome the above-described problems. For example, in the image recording apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 179884/1999, the optical axis of the detection mechanism is set in a direction crossing the arrangement direction of the nozzles. Therefore, with the detection mechanism 160′ being fixed, the beam B is caused to cross the flying path of the ink of the respective nozzles sequentially by moving the carriage as shown in FIG. 14A. Therefore, all the nozzles 121 can be assuredly caused to cross the optical axis of the beam B, thereby enabling detection of an ejection state.
Incidentally, since the photo detector 163 detects an ejection state based on a quantity of light when receiving the beam B, the ejection state of each nozzle can not be correctly detected if a plurality of the ink droplets have passed through the beam B at the same time. Therefore, in the image recording apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 179884/1999, the detection mechanism 160′ has an angle of the optical axis adjusted with respect to the arrangement direction of the nozzle column as shown in FIG. 14B. Specifically, when including a plurality of nozzle columns, the light source 162 have an angle of the optical axis adjusted with respect to the arrangement direction of the nozzle columns in such a manner that the ink flying paths of a plurality of the nozzles do not cross the beam B at the same time. More specifically, an angle θ of the optical axis relative to the arrangement direction of the nozzle columns must have the relationship of the follow expression 1:l×tan θ<w  (Expression 1)
w: gap between nozzle columns N1 and N2 adjacent to each other
l: length of the nozzle columns N1 and N2
In general, when the gap w is increased, a width of the entire image recording apparatus becomes large. Therefore, the smaller gap w is good. Based on this restriction, the angle θ is generally selected to be a value smaller than 45 degrees.
Description will now be given as to the case where the recording head is moved for one cycle of the ink ejection cycle at a moving speed in image recording when the optical head crosses the nozzle 121_1 at the end of the recording head in the image recording apparatus with reference to FIG. 14C.
It is to be noted that the recording head 120 moves for the same distance as the interval of the nozzles when the recording head is moved in one cycle of the ink ejection cycle at the moving speed in image recording as described above. Therefore, when the recording head 120 is moved for a time corresponding to the one cycle, it moves for the same distance as the interval of the nozzles along the main scan direction. In FIG. 14, the recording head 120 after movement is indicated by a broken line. Therefore, when the angle θ is set smaller than 45 degrees, the nozzle 121_2 of the recording head 120 after movement moves beyond the beam B in the main scan direction. Therefore, the image recording apparatus must lower the moving speed of the recording head below the moving speed in image recording in order to detect the ejection state of all the nozzles 121. Accordingly, the image recording apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 179884/1999 requires a complicated mechanism in order to slow the speed of the recording head, and the image recording speed including the inspection time is decreased.
In the image recording apparatus including the detection mechanism, the image recording apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 188853/1999 is proposed in order to overcome the above-described problems. In the image recording apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 188853/1999, the optical axis of the detection mechanism is set in a direction crossing the arrangement direction of the nozzles as similar to the image recording apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 179884/1999. Additionally, the angle θ of the optical axis of the detection mechanism with respect to the nozzle arrangement direction also has the relationship similar to that shown in the expression 1. However, in the image recording apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 188853/1999, when the recording head is moved for a time corresponding to one cycle of the ink ejection cycle, the angle of the optical axis is adjusted in such a manner that, after at least one nozzle 121 has passed through the optical axis of the detection mechanism, another nozzle 121 different from the nozzle having passed through the optical axis is arranged on the optical axis. More specifically, as shown in FIG. 15, the nozzle 121_1 indicated by a solid line is placed at a position crossing the optical axis before movement of the recording head 120. The recording head 120 is moved for a time corresponding to one cycle of the ink ejection cycle. After this movement, as indicated by a broken line, the nozzles 121_2 and 121_3 move in the main scan direction beyond the beam B. However, the nozzle 121_4 indicated by the broken line is arranged on the optical axis. In this manner, the image recording apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 188853/1999 can detect the ejection state of the nozzles even if the carriage is moved at the moving speed in the regular image recording mode.
However, as described above, the image recording apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 188853/1999 skips at least one nozzle every cycle of the ink ejection cycle and detects the ejection state of the next nozzle. Therefore, the recording head must be scanned for a plurality of number of times in order to inspect all the nozzles. Accordingly, in this image recording apparatus, the speed for recording an image including the inspection time is still slow.
Further, in recent years, improvement of the image quality is demanded in the image recording apparatus, and elongation of the recording head or minimization of the interval of the nozzles is advanced. In this case, in the image recording apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 188853/1999, the above-described angle θ must be further reduced. Therefore, in this image recording apparatus, the recording speed must be further slowed, or the number of times of scanning the recording head must be increased. Thus, arrangement of the optical axis in the detection mechanism must be highly accurately adjusted, and hence manufacture may become difficult.
In view of the above-described problems, there is desired an image recording apparatus having a detection mechanism which can detect an ejection state of each nozzle at a high speed and does not require sophisticated positional adjustment of the optical axis of the detection mechanism and the nozzles.